1. Field of the Invention
This invention relates to a projection cathode-ray tube for use in a projection type television apparatus, and more particularly to an improvement of brightness within small angles to the normal on a display screen of the projection cathode-ray tube. The cathode-ray tube includes an optical multilayered interference film interposed between a face plate and a phosphor screen.
2. Description of the Related Arts
In FIG. 3, a projection television apparatus is diagrammatically illustrated. A projection television apparatus 10 comprises three monochromatic cathode-ray tubes 11 which emit light rays in red, green, and blue, respectively. Monochromatic images produced on a luminescent screen of respective cathode-ray tubes are enlarged and projected on a screen 13, located at a given distance ahead, through projection lens systems 12 disposed in close proximity to the luminescent screen. Thereby, an enlarged color image is reproduced on the screen 13.
FIG. 4 is a partially cutaway sectional side elevation view showing a conventional projection cathode-ray tube. In a cathode-ray tube 11, an inner surface of a face plate 1 is curved towards an evacuated side of the cathode-ray tube. On the inner surface of the face plate 1 are superimposed, one over another, in the order of a multilayered interference film 2, a phosphor layer 3, and a thin aluminum (Al) film 4. The phosphor layer 3 is excited by an electron beam 6 being emitted from an electron gun 5, thereby emanating a monochromatic light.
Japanese Patent Publication Laid-open No. SHO-55-150532 does not disclose a projection cathode-ray tube, but discloses a direct viewing type picture tube comprising a multilayered interference film interposed between a face plate and a phosphor layer. This multilayered interference film is composed of a plurality of layers piled up alternately from an optically transparent material layer having a high refractive index and an optically transparent material layer having a low refractive index.
This interference film allows light rays that are at small angles to the normal pass through, while light rays at large angles to the normal are reflected back towards the phosphor layer.
Thus reflected light rays are again diffusely scattered by phosphor particles, and the light rays reflected within small angles to the normal on the phosphor screen can only travel through the interference film. Accordingly, a halo occurring on the display surface of the cathode-ray tube is prevented.
U.S. Pat. No. 4,642,695 discloses a cathode-ray tube, for use in a projection television apparatus, having a multilayered interference film interposed between the face plate and the phosphor layer. The presence of the interference film results in the convergence of the light rays within +/-30 degrees to the normal on the display surface. As a result, the total quantity of light rays being gathered into the projection lens system is increased, enhancing the brightness on the screen.
Japanese Patent Publication Laid-open No. SHO-61-39349 particularly describes the structure and characteristics of a multilayered interference film for use in a cathode-ray tube, and states that most light rays emitted forwardly of the cathode-ray tube are gathered into a lens system having an acceptance angle between 25 to 30 degrees to the normal on the display screen without any substantial loss.
Further, Japanese Patent Publication Laid-open No. SHO-61-273837 teaches that by using a cathode-ray tube including a face plate having a curvature angle within 5 to 25 degrees to the normal, that is, convex towards the evacuated side of the cathode-ray tube, brightness at the corners and edge of the screen is improved.
Here, the curvature angle is an angle formed between an axis perpendicular to the center of the display screen and a straight line orthogonal to the display screen and located at a distal end from the center of the display screen. In short, the difference in brightness between the periphery and center of the screen can be decreased.
In addition, from Japanese Patent Publication Laid-open No. Hei-1-95450, it is found that since the multilayered interference film can reduce unnecessary components out of the total spectra emanated from the phosphor layer, it becomes possible to change the chromaticity of emitted light from the cathode-ray tube.
As described above, with use of the multilayered interference film, a variety of studies has been made to improve the brightness and chromaticity on the display screen of the projection television apparatus.
Meanwhile, constant efforts have been made to reduce a brightness deterioration with time regarding the structure of the phosphor layer and the shape of the luminescent material of the cathode-ray tube relative to the direct-viewing type cathode-ray tube and the projection cathode-ray tube without the interference film.
A correlation between the thickness of the phosphor layer, the diameter of phosphor particles and brightness is theoretically discussed in an article on pages 894-899 of a journal entitled "J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY" Vol. 121, 1974.
In this article, it is described that a maximum brightness is produced when the thickness of the phosphor layer is 1.4 times as thick as the diameter of the phosphor particles. These results are experimentally confirmed in an article on pages 478-485 of a journal entitled "IEEE Transactions on Consumer Electronics" Vol. CE-27, No. 3, August, 1981.
Further, Japanese Patent Publications Laid-open No. SHO-49-43075 and SHO-49-43076 disclose a surface processing technique of phosphor particles consisting of zinc sulphide compounds by the use of a phosphoric compound material and a surface processing technique of phosphor particles consisting of zinc sulphide cadmium compounds by the use of the same material.
These surface processing techniques contribute to the improvement of the dispersibility and adhesiveness of the phosphor particles and the prevention of brightness deterioration of the phosphor layer due to an exposure to heat treatment of manufacturing processes of the cathode-ray tube and due to contamination by impurities.
By virtue of these improvements in brightness and efforts for preventing the brightness deterioration of the cathode-ray tube as being set forth in the above, brightness of the projection televisions of the size of 40" to 70" has been enhanced year by year, whereby an image having sufficient clarity can be reproduced under normal house lighting.
However, in order to realize the screen size of the projection television apparatus up to 100" or more, it may not be said that a sufficient brightness has already been achieved. Accordingly, a proper image cannot be obtained without darkening the room. Namely, brightness of the cathode-ray tube can be improved still further.